This invention relates to the manufacture of a fiber reinforced composite article using resin transfer molding, and, and more particularly, to an improved method of adhering or bonding the layers of composite fabrics made of fibrous material to provide an improved composite preform.
Because of its high strength to weight ratio, fiber reinforced composite structures have become attractive for aerospace applications, such as parts for airframes and propulsion power plants including gas turbine engines. These structures, made from fiber-reinforced materials, such as yams, fibers, filaments and the like, have substantial rigidity and other properties, which make them desirable for many applications. Certain of the composite fabrics comprising these structures even have sufficient rigidity and strength so that they can be formed into components previously made from metals. For example, many composite fabrics have sufficient strength, rigidity, impact resistance and other desirable properties to make them useful materials for gas turbine blade members, for example, fan blades for jet engines.
Resin transfer molding (RTM) is one process used to make these fiber-reinforced composite structures. Layers of reinforcing material of a predetermined shape, known as preforms, are loaded into a mold, and a thermosetting or EM wavelength activated resin binder is injected into the mold and cured by known procedures to provide a finished composite piece. Molding of such parts has been relatively time consuming and labor intensive, in part, because of the need to properly position and align the layers of preform accurately in the mold.
In order to mechanically hold the preform layers of substantially dry reinforcing fabrics and to prevent damage to the yams, fibers and/or filaments in the stacked assembly of such dry fabric layers during various operations, a resin, usually a thermoplastic, is used as a binder or xe2x80x9ctackifierxe2x80x9d to stiffen the reinforcing fabric and hold it in place before the molding process begins.
Methods of impregnating fiber with a tackifier are well known. For example, in U.S Pat. No. 5,382,148, a tackifier having two distinct curing stages is continuously applied by way of a sprayer as a spray to a glass fiber mat to effectuate a thin, penetrating random disposition of droplets. Upon curing of both stages, a rigid preform is produced.
U.S. Pat. No. 5,041,260 discloses a spray-up process, wherein chopped glass roving or other chopped fiber reinforcement material is sprayed onto a forming mandrel from a chopper gun. The fibers may be resin coated or a small amount of resin may be introduced into the stream of chopped fibers to cause it to be retained on the mandrel. A web is then placed in direct contact with and encapsulates at least a portion of the sprayed reinforcement material.
Similar methods, such as U.S. Pat. No. 5,688,467 utilize a fiber nozzle to blow a stream of chopped fibers onto a screen, and a tackifier nozzle to continuously spray tackifier into the fiber stream. A ducted fan creates a vacuum and helps to draw and hold the fibers onto the screen.
Other prior art methods, such as disclosed in U.S. Pat. No. 5,698,318, set forth processes in which individual fiber plies are sprinkled with a solid powder of the tackifier. The powder is heated to fuse it onto the surface of the substrate and then cooled to solidify it. The different plies can be stacked together, heated to fuse the plies together, and then cooled leaving a preform to be placed into a mold for use in an ordinary RTM process. The tackifier, in powder form, is not applied to the fiber plies in any predetermined discontinuous pattern, and preferably is applied evenly across the plies.
The RTM process consists of placing these preforms of predetermined shape into a mold. The mold is closed, and vacuum may or may not be applied to the closed mold. A liquid resin is injected into the mold, the resin injection process continuing until the mold is completely filled and no resin free areas are left in the mold. At the completion of the injection process, the resin is then cured in the case of reactive resin systems, or it is cooled in the case of thermoplastic resin systems. Upon cooling, the mold is opened and the part is removed.
Critical to the success of the RTM process is the construction of the preform. The preform must be constructed in such a manner as to allow for handling and placement into the RTM mold. Stabilization of the preform is achieved by applying tackifier to the surface of each layer of reinforcing fibers. The amount of tackifier and its location on the layer of reinforcing fibers can greatly affect the handleability, the stackability and the injectability of the preform.
One problem with the above described methods of applying the tackifier is that powder systems can result in a layer of powder that will not go into solution in the RTM resin, significantly reducing the mechanical properties in the final part. Continuous film spraying methods may leave too much resin on the layer of reinforcing fibers resulting in reduced RTM resin injection capability thereby weakening the final part, lack of flexibility in the preform and high potential for void formation in the final part.
There is, therefor, a continuous need for improved designs for preform manufacturing methods that will increase both handling capability and quality of the final part. The present invention fulfills this need, and further provides related advantages.
The present invention provides for a method of manufacturing a preform that will increase both the handling capabilities of the preform and the final part quality. The invention consists of applying a patterned discontinuous layer of liquid resin over one or both sides of a layer of reinforcing fibers. The resin film can consist of a homogeneous liquid, an aqueous dispersion of resin powder or particles or a resin applied at an elevated temperature at which it is fluid, but which solidifies on cooling to ambient temperatures.
In one embodiment, there is present a roller having a patterned surface. Liquid resin is applied to the surface of the patterned roller. The layer of reinforcing fibers is passed over the roller, contacting the surface so that the resin from the pattern on the surface of the roller is transferred to the layer of reinforcing fibers. The contact of the roller with the layer of reinforcing fibers is sufficient to force some of the resin between the fibers themselves. After the resin is applied, the layer of reinforcing fibers with resin is passed through an additional process to cause the resin to turn into a solid flexible coating. The layer of fiber is then wrapped onto a cardboard roll or similar type material and packaged for storage or subsequent shipment.
One advantage of the present invention is that the patterned roller provides a reduced amount of tackifier resin, but sufficient resin to adhere layers for handling purposes and placing them in a preform, resulting in a significant decrease in the probability of the tackifier not going into solution in the RTM resin.
Another advantage is that decreasing the amount of resin applied to the layer of reinforcing fibers increases the preform flexibility. This is especially important when subsequent process operations are needed on the preform to achieve net shape or near net shape prior to loading into the RTM molding tool.
Still another advantage is that the method of applying the tackifier of the present invention can be used with any type of fiber, such as carbon fibers, graphic fibers, glass fibers, aramid fibers or any combination thereof.
Yet another advantage is that the present method has the flexibility to use resins that are liquid at room temperature as well as those resins that become liquid at elevated temperature.
The present invention also has the flexibility to be applied to one or both sides of the layer of reinforcing fibers and a further advantage in that the resin chemistry can be epoxy based, typically with curative bis-maleimides, and thermoplastic such as a polyester, or an aqueous system such as polyvinyl alcohol.
Yet another advantage of the present invention is that the discontinuous film of tackifier resin applies by the patterned roller to the layer of reinforcing fibers enhances the subsequent resin flow through the fiber preform during the RTM process.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying figures which illustrate, by way of example, the principles of the invention.